U.S. patent application number 13/125227 was filed with the patent office on 2011-08-11 for image display device and head-mounted display.
This patent application is currently assigned to Konica Minolta Opto, Inc.. Invention is credited to Yoshie Shimizu, Yasushi Tanijiri.
Application Number | 20110194163 13/125227 |
Document ID | / |
Family ID | 42225708 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110194163 |
Kind Code |
A1 |
Shimizu; Yoshie ; et
al. |
August 11, 2011 |
IMAGE DISPLAY DEVICE AND HEAD-MOUNTED DISPLAY
Abstract
A total reflection surface and a holographic optical element
(HOE) surface are formed on the same surface (S3) in an eyepiece
prism (15). In this structure, a smaller reflection angle can be
set at an HOE (16) compared to a structure with the surface formed
separately, and the HOE surface of the surface (S3) can be set in
the direction parallel to a surface (S2). Thus, even in a structure
in which at least a portion of the light flux of the image light
fully reflected by the surface (S3) is incident on an affixing
region (R1) of a hologram photosensitive material (16a), that
portion of the light can be prevented from falling incident on the
optical pupil (E) as ghost light. Consequently, in order to prevent
the generation of ghost light, an optical path margin no longer
needs to be provided between the diffraction and reflection region
of the HOE (16) and the total reflection region of the image light;
and the eyepiece prism (15) can be thinned by that amount. In
addition, since a smaller reflection angle can be set at the HOE
(16), the color dispersion caused by diffraction by the HOE (16)
can also be reduced, and the image quality can be maintained.
Inventors: |
Shimizu; Yoshie;
(Ibaraki-shi, JP) ; Tanijiri; Yasushi;
(Osakasayama-shi, JP) |
Assignee: |
Konica Minolta Opto, Inc.
Hachioji-shi, Tokyo
JP
|
Family ID: |
42225708 |
Appl. No.: |
13/125227 |
Filed: |
November 25, 2009 |
PCT Filed: |
November 25, 2009 |
PCT NO: |
PCT/JP2009/069831 |
371 Date: |
April 20, 2011 |
Current U.S.
Class: |
359/15 |
Current CPC
Class: |
G02B 27/0172 20130101;
G03H 2270/55 20130101; G02B 2027/0174 20130101; G02B 2027/012
20130101; G02B 5/32 20130101; G03H 2270/21 20130101; G02B 2027/0161
20130101; G02B 6/00 20130101; G02B 2027/011 20130101 |
Class at
Publication: |
359/15 |
International
Class: |
G02B 5/32 20060101
G02B005/32 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 26, 2008 |
JP |
2008-300502 |
Claims
1.-11. (canceled)
12. An image display device comprising: a display element for
displaying an image; and an eyepiece optical system for directing
image light from the display element to an optical pupil, the
eyepiece optical system comprising: an eyepiece prism having a
surface S1 on which the image light is incident, a surface S2 which
is disposed toward the optical pupil, and a surface S3 which is
disposed opposite from the surface S2, and a volume-phase
reflective holographic optical element formed on part of the
surface S3, wherein the image light from the display element enters
the eyepiece prism through the surface S1, is then totally
reflected on the surface S3 at least once, is then totally
reflected on the surface S2, and is then diffraction-reflected by
the holographic optical element on the surface S3 so as to be
directed to the optical pupil, when an axis optically connecting a
center of a display screen of the display element to a center of
the optical pupil is defined as an optical axis, and a plane
including an optical axis of light incident on the surface S3 and
an optical axis of light emergent from the surface S3 is defined as
an optical axis incidence plane, then the eyepiece prism is shaped
symmetrically about the optical axis incidence plane and is so
shaped that a distance between the surfaces S2 and S3 continuously
decreases away from the surface S1, and at least part of a beam of
the image light totally reflected on the surface S3 is incident on
an attachment region of an hologram photosensitive material where
the holographic optical element is produced.
13. The image display device according to claim 12, wherein an
effective diffraction region within the attachment region of the
hologram photosensitive material where the holographic optical
element is, or the holographic optical elements are, produced is
set by restricting an exposed region within the attachment
region.
14. The image display device according to claim 12, wherein the
attachment region of the hologram photosensitive material where the
holographic optical element is produced includes a
diffraction-reflection region and a total reflection region for the
image light on the surface S3.
15. The image display device according to claim 12, wherein the
surface S3 has a curvature only on the optical axis incidence
plane.
16. The image display device according to claim 12, further
comprising a correction prism for canceling refraction of light of
an outside world image in the eyepiece prism, wherein any joint
line along which the eyepiece prism and the correction prism are
joined together is located on a side face that intersects a surface
through which the light of the outside world image is
transmitted.
17. The image display device according to claim 12, further
comprising a correction prism for canceling refraction of light of
an outside world image in the eyepiece prism, wherein at least one
of the eyepiece prism and the correction prism comprises a
positioning portion for joining together the eyepiece prism and the
correction prism at a predetermined interval from each other with a
layer of air in between.
18. The image display device according to claim 12, wherein the
surface S3 is a flat surface.
19. An image display device comprising: a display element for
displaying an image; and an eyepiece optical system for directing
image light from the display element to an optical pupil, the
eyepiece optical system comprising: an eyepiece prism having a
surface S1 on which the image light is incident, a surface S2 which
is disposed toward the optical pupil, and a surface S3 which is
disposed opposite from the surface S2, a first volume-phase
reflective holographic optical element formed on the surface S3,
and a second volume-phase reflective holographic optical element
formed on the surface S3, the image light from the display element
enters the eyepiece prism through the surface S1, is then
diffraction-reflected by the first holographic optical element on
the surface S3 at least once, is then totally reflected on the
surface S2, and is then diffraction- reflected by the second
holographic optical element on the surface S3 so as to be directed
to the optical pupil, when an axis optically connecting a center of
a display screen of the display element to a center of the optical
pupil is defined as an optical axis, and a plane including an
optical axis of light incident on the surface S3 and an optical
axis of light emergent from the surface S3 is defined as an optical
axis incidence plane, then the eyepiece prism is shaped
symmetrically about the optical axis incidence plane and is so
shaped that a distance between the surfaces S2 and S3 continuously
decreases away from the surface S1, and part of a beam of the image
light diffraction-reflected by the first holographic optical
element is incident on a diffraction-reflection region of the
second holographic optical element.
20. The image display device according to claim 19, wherein an
effective diffraction region within the attachment region of the
hologram photosensitive material where the holographic optical
element is, or the holographic optical elements are, produced is
set by restricting an exposed region within the attachment
region.
21. The image display device according to claim 19, wherein the
attachment region of the hologram photosensitive material where the
second holographic optical element is produced includes the
diffraction-reflection region of the second holographic optical
element and a diffraction-reflection region of the first
holographic optical element.
22. The image display device according to claim 21, wherein, in
part of the hologram photosensitive material, interference fringes
for the first holographic optical element and interference fringes
for the second holographic optical element are both formed by
multiple exposure.
23. The image display device according to claim 19, wherein the
surface S3 has a curvature only on the optical axis incidence
plane.
24. The image display device according to claim 19, further
comprising a correction prism for canceling refraction of light of
an outside world image in the eyepiece prism, wherein any joint
line along which the eyepiece prism and the correction prism are
joined together is located on a side face that intersects a surface
through which the light of the outside world image is
transmitted.
25. The image display device according to claim 19, further
comprising a correction prism for canceling refraction of light of
an outside world image in the eyepiece prism, wherein at least one
of the eyepiece prism and the correction prism comprises a
positioning portion for joining together the eyepiece prism and the
correction prism at a predetermined interval from each other with a
layer of air in between.
26. The image display device according to claim 19, wherein the
surface S3 is a flat surface.
27. A head-mounted display comprising: an image display device; and
a supporting mechanism for supporting the image display device in
front of an eye of a viewer, the image display device comprising: a
display element for displaying an image; and an eyepiece optical
system for directing image light from the display element to an
optical pupil, the eyepiece optical system comprising: an eyepiece
prism having a surface S1 on which the image light is incident, a
surface S2 which is disposed toward the optical pupil, and a
surface S3 which is disposed opposite from the surface S2, and a
volume-phase reflective holographic optical element formed on part
of the surface S3, wherein the image light from the display element
enters the eyepiece prism through the surface S1, is then totally
reflected on the surface S3 at least once, is then totally
reflected on the surface S2, and is then diffraction-reflected by
the holographic optical element on the surface S3 so as to be
directed to the optical pupil, when an axis optically connecting a
center of a display screen of the display element to a center of
the optical pupil is defined as an optical axis, and a plane
including an optical axis of light incident on the surface S3 and
an optical axis of light emergent from the surface S3 is defined as
an optical axis incidence plane, then the eyepiece prism is shaped
symmetrically about the optical axis incidence plane and is so
shaped that a distance between the surfaces S2 and S3 continuously
decreases away from the surface S1, and at least part of a beam of
the image light totally reflected on the surface S3 is incident on
an attachment region of an hologram photosensitive material where
the holographic optical element is produced.
28. The head-mounted display according to claim 27, wherein the
attachment region of the hologram photosensitive material where the
holographic optical element is produced includes a
diffraction-reflection region and a total reflection region for the
image light on the surface S3.
29. A head-mounted display comprising: an image display device; and
a supporting mechanism for supporting the image display device in
front of an eye of a viewer, the image display device comprising: a
display element for displaying an image; and an eyepiece optical
system for directing image light from the display element to an
optical pupil, the eyepiece optical system comprising: an eyepiece
prism having a surface S1 on which the image light is incident, a
surface S2 which is disposed toward the optical pupil, and a
surface S3 which is disposed opposite from the surface S2, a first
volume-phase reflective holographic optical element formed on the
surface S3, and a second volume-phase reflective holographic
optical element formed on the surface S3, the image light from the
display element enters the eyepiece prism through the surface S1,
is then diffraction-reflected by the first holographic optical
element on the surface S3 at least once, is then totally reflected
on the surface S2, and is then diffraction-reflected by the second
holographic optical element on the surface S3 so as to be directed
to the optical pupil, when an axis optically connecting a center of
a display screen of the display element to a center of the optical
pupil is defined as an optical axis, and a plane including an
optical axis of light incident on the surface S3 and an optical
axis of light emergent from the surface S3 is defined as an optical
axis incidence plane, then the eyepiece prism is shaped
symmetrically about the optical axis incidence plane and is so
shaped that a distance between the surfaces S2 and S3 continuously
decreases away from the surface S1, and part of a beam of the image
light diffraction-reflected by the first holographic optical
element is incident on a diffraction-reflection region of the
second holographic optical element.
30. The head-mounted display according to claim 29, wherein the
attachment region of the hologram photosensitive material where the
second holographic optical element is produced includes the
diffraction-reflection region of the second holographic optical
element and a diffraction-reflection region of the first
holographic optical element.
Description
TECHNICAL FIELD
[0001] The present invention relates to an image display device
that directs image light from a display element through an eyepiece
optical system to an optical pupil in order to thereby allow a
viewer to observe a displayed image (virtual image) at the position
of the optical pupil. The present invention also relates to a
head-mounted display (hereinafter also referred to as a HMD).
BACKGROUND ART
[0002] An image display device that directs image light from a
display element through an eyepiece optical system to an optical
pupil is disclosed, for example, in Patent Document 1 listed below.
In this image display device, the eyepiece optical system includes
an eyepiece prism, which has an entrance surface S11, two opposite
surfaces S12 and S13 disposed opposite from each other, and a HOE
surface S14 on which a hologram optical element is formed. Part of
one opposite surface S12 serves as an exit surface as well. With
this construction, the image light from a display element enters
the eyepiece prism through the entrance surface S11, is then
directed, by being totally reflected between the two opposite
surfaces S12 and S13, to the HOE surface S14, is then
diffraction-reflected on the HOE surface S14, and is thus directed,
through the exit surface, to the optical pupil. This allows a
viewer to observe, at the position of the optical pupil, a virtual
image of the image displayed on the display element.
LIST OF CITATIONS
Patent Literature
[0003] Patent Document 1: JP-A-2004-61731
SUMMARY OF INVENTION
Technical Problem
[0004] Inconveniently, however, in the image display device
disclosed in Patent Document 1, the opposite surfaces S12 and S13
are disposed parallel to each other, and in addition one opposite
surface S13 and the HOE surface S14 are formed as separate surfaces
(discontinuous surfaces); moreover, the HOE surface S14 is so
inclined that its distance from the opposite surface S12
continuously decreases away from the entrance surface S11. With
this construction, if part of the rays that should be incident on
the opposite surface S13 are, due to an error in the inclination of
a surface in the eyepiece optical system or a displacement of the
display element, diffracted on the HOE surface S14, they become
ghost light and are incident on the optical pupil. To prevent this,
it is necessary to ensure that the beam incident on the opposite
surface S13 is separated from the beam incident on the HOE surface
S14. To achieve that, it is necessary to secure a space (optical
path margin) for separating those beams near the boundary between
the opposite surface S13 and the HOE surface S14.
Disadvantageously, securing this optical path margin makes the
eyepiece optical system thicker.
[0005] Specifically, as shown in FIG. 14, in the eyepiece optical
system 101, if a ray L11 at the bottom end of the image region
(screen) which is incident on the optical pupil E at its bottom end
is, due to an error in inclination etc. mentioned above, incident
on the HOE surface S14, on which a HOE 102 is formed, that ray L1,
since its angle of incidence is close to the angle of incidence of
a ray L12 at the top end of the image region which is incident on
the optical pupil E at its top end, is incident on the optical
pupil E as ghost light. To suppress occurrence of such ghost light,
it is necessary to secure an optical path margin P for separating
the rays L11 incident on the opposite surface S13, which is a total
reflection surface, from the rays L12 incident on the HOE surface
S14. And securing the optical path margin P makes the eyepiece
optical system 101 accordingly thicker.
[0006] On the other hand, if the angle of diffraction of the image
light on the HOE 102 is large, the color dispersion caused by
diffraction on the HOE 102 is large, leading to lower image
quality. This too, therefore, needs to be taken into consideration
in attempting to make the eyepiece optical system 101 slim.
[0007] The present invention has been made to overcome the
inconveniences discussed above, and it is an object of the
invention to provide an image display device that, while
maintaining satisfactory image quality, prevents occurrence of
ghost light and in addition permits an eyepiece prism to be made
slim, and to provide a HMD incorporating such an image display
device.
Solution to Problem
[0008] According to one aspect of the invention, an image display
device includes: a display element for displaying an image; and an
eyepiece optical system for directing t image light from the
display element to an optical pupil, the eyepiece optical system
including an eyepiece prism having a surface S1 on which the image
light is incident, a surface S2 which is disposed toward the
optical pupil, and a surface S3 which is disposed opposite from the
surface S2. Here, on part of the surface S3, a volume-phase
reflective holographic optical element is formed; the image light
from the display element enters the eyepiece prism through the
surface S1, is then totally reflected on the surface S3 at least
once, is then totally reflected on the surface S2, and is then
diffraction-reflected by the holographic optical element on the
surface S3 so as to be directed to the optical pupil; when an axis
optically connecting the center of the display screen of the
display element to the center of the optical pupil is defined as
the optical axis, and a plane including the optical axis of the
light incident on the surface S3 and the optical axis of the light
emergent from the surface S3 is defined as the optical axis
incidence plane, then the eyepiece prism is shaped symmetrically
about the optical axis incidence plane and is so shaped that the
distance between the surfaces S2 and S3 continuously decreases away
from the surface S1; and at least part of the beam of the image
light totally reflected on the surface S3 is incident on the
attachment region of an hologram photosensitive material where the
holographic optical element is produced.
[0009] According to another aspect of the invention, an image
display device includes: a display element for displaying an image;
and an eyepiece optical system for directing image light from the
display element to an optical pupil, the eyepiece optical system
including an eyepiece prism having a surface S1 on which the image
light is incident, a surface S2 which is disposed toward the
optical pupil, and a surface S3 which is disposed opposite from the
surface S2. Here, on the surface S3, a first volume-phase
reflective holographic optical element and a second volume-phase
reflective holographic optical element are formed; the image light
from the display element enters the eyepiece prism through the
surface S1, is then diffraction-reflected by the first holographic
optical element on the surface S3 at least once, is then totally
reflected on the surface S2, and is then diffraction-reflected by
the second holographic optical element on the surface S3 so as to
be directed to the optical pupil; when an axis optically connecting
the center of the display screen of the display element to the
center of the optical pupil is defined as the optical axis, and a
plane including the optical axis of light incident on the surface
S3 and the optical axis of light emergent from the surface S3 is
defined as the optical axis incidence plane, then the eyepiece
prism is shaped symmetrically about the optical axis incidence
plane and is so shaped that the distance between the surfaces S2
and S3 continuously decreases away from the surface S1; and part of
the beam of the image light diffraction-reflected by the first
holographic optical element is incident on a diffraction-reflection
region of the second holographic optical element.
[0010] In an image display device according to the invention, it is
preferable that the effective diffraction region within the
attachment region of the hologram photosensitive material where the
holographic optical element is, or the holographic optical elements
are, produced be set by restricting the exposed region within the
attachment region.
[0011] In an image display device according to the invention, the
attachment region of the hologram photosensitive material where the
holographic optical element is produced may include a
diffraction-reflection region and a total reflection region for the
image light on the surface S3.
[0012] In an image display device according to the invention, the
attachment region of the hologram photosensitive material where the
second holographic optical element is produced may include the
diffraction-reflection region of the second holographic optical
element and a diffraction-reflection region of the first
holographic optical element.
[0013] In an image display device according to the invention, in
part of the hologram photosensitive material, interference fringes
for the first holographic optical element and interference fringes
for the second holographic optical element may both be formed by
multiple exposure.
[0014] In an image display device according to the invention, the
surface S3 may have a curvature only on the optical axis incidence
plane.
[0015] In an image display device according to the invention, it is
preferable that it further include a correction prism for canceling
refraction of light of an outside world image in the eyepiece
prism, and that any joint line along which the eyepiece prism and
the correction prism are joined together be located on a side face
that intersects a surface through which the light of the outside
world image is transmitted.
[0016] In an image display device according to the invention, it is
preferable that it further include a correction prism for canceling
the refraction of light of an outside world image in the eyepiece
prism, and that at least one of the eyepiece prism and the
correction prism include a positioning portion for joining together
the eyepiece prism and the correction prism at a predetermined
interval from each other with a layer of air in between.
[0017] In an image display device according to the invention, the
surface S3 may be a flat surface.
[0018] According to yet another aspect of the invention, a
head-mounted display includes: an image display device according to
the invention as described above; and support means for supporting
the image display device in front of the eye of a viewer.
Advantageous Effects of the Invention
[0019] According to the invention, the eyepiece prism has a total
reflection surface and a HOE surface formed on the same surface S3,
and in addition is so shaped that the distance between the surfaces
S2 and S3 continuously decreases away from the surface S1. With
this construction, as compared with one where total reflection
surfaces are disposed parallel to each other and in addition a
total-reflection part of the surface S3 and a HOE part are formed
separately, even when a HOE surface is set up in a direction
parallel to the surface S2, it is possible to reduce the angle of
incidence of the image light on the HOE, and thus to reduce the
angle of reflection (diffraction) on the HOE. By reducing the angle
of diffraction on the HOE, it is possible to keep the color
dispersion caused by diffraction small, and thus it is possible,
while maintaining satisfactory image quality, to make the eyepiece
prism slim.
[0020] Moreover, owing to the eyepiece prism being so shaped that
the distance between the surfaces S2 and S3 continuously decreases
away from the surface S1, even in a construction where at least
part of the beam of the image light totally reflected on the
surface S3 is incident on the attachment region of the hologram
photosensitive material, since the angle of incidence differs
between ghost light and the light diffracted on the HOE, it is
possible, with the angle selectivity of the HOE, to prevent ghost
light from being incident on the optical pupil.
[0021] It is thus no longer necessary, for the purpose of reducing
the angle of incidence on a HOE, or with a view to preventing
occurrence of ghost light, to give the HOE a large inclination, or
secure an optical path margin. This makes it possible to make the
eyepiece prism accordingly slimmer. That is, with the construction
described above, it is possible, while maintaining satisfactory
image quality, to prevent occurrence of ghost light and in addition
make the eyepiece prism slim.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIG. 1 shows, with enlargement, the construction of an image
display device according to one embodiment of the invention, and is
a sectional view showing, with enlargement, part A in FIG. 2.
[0023] FIG. 2 is a sectional view showing an outline of the
construction of the image display device.
[0024] FIG. 3 is a diagram illustrating the spectral intensity
distribution of a light source in the image display device.
[0025] FIG. 4 is a diagram illustrating the wavelength dependence
of the diffraction efficiency on an HOE in the image display
device.
[0026] FIG. 5 is a schematic sectional view of an eyepiece prism in
an eyepiece optical system in the image display device.
[0027] FIG. 6 is a perspective view of an image display device
incorporating another eyepiece prism.
[0028] FIG. 7 is a sectional view showing an outline of the
construction of a production optical system for producing the
HOE.
[0029] FIG. 8 is a sectional view showing another construction of
the image display device.
[0030] FIG. 9 is a sectional view showing yet another construction
of the image display device.
[0031] FIG. 10 is a sectional view showing an outline of the
construction of an image display device according to another
embodiment of the invention.
[0032] FIG. 11 is a sectional view showing an outline of the
construction of an image display device according to yet another
embodiment of the invention.
[0033] FIG. 12 is a sectional view showing another construction of
the image display device.
[0034] FIG. 13 is a perspective view showing an outline of the
construction of an HMD according to still another embodiment of the
invention.
[0035] FIG. 14 is a sectional view of a relevant part of a
conventional image display device.
DESCRIPTION OF EMBODIMENTS
Embodiment 1
[0036] An embodiment of the invention will be described below with
reference to the accompanying drawings.
[0037] (Image Display Device)
[0038] FIG. 2 is a sectional view showing an outline of the
construction of an image display device 1 according this
embodiment. This image display device 1 generates an image to
present it as a virtual image to a viewer and in addition permits
the viewer to observe an outside world image on a see-through
basis. The image display device 1 includes a light source 11, an
illumination optical system 12, a display element 13, and an
eyepiece optical system 14.
[0039] For convenience's sake, the following description uses the
following terminology. The axis that optically connects the center
of the light source 11 to the center of the display screen (image
region) on the display element 13 to the center of the optical
pupil (exit pupil) formed by the eyepiece optical system 14 is
referred to as the optical axis. The direction of the optical axis
from the light source 11 to the optical pupil E, as it is when
straightened, is taken as the Z direction. The direction
perpendicular to the optical axis incidence plane of a surface S3
of an eyepiece prism 15, which will be described later, is taken as
the X direction, and the direction perpendicular to the ZX plane is
taken as the Y direction. It should be noted that the optical axis
incidence plane of the surface S3 denotes the plane on which lie
both the optical axis of the light incident on the surface S3 and
the optical axis of the light reflected from the surface S3; that
is, it denotes the YZ plane.
[0040] The light source 11 is, for example, composed of
light-emitting diodes (LEDs) that emit light of wavelengths
corresponding to three primary colors, namely R (red), G (green),
and B (blue). FIG. 3 is a diagram illustrating the spectral
intensity distribution of the light source 11, that is, the
relationship between the wavelength and intensity of the light
emitted. The light source 11 emits light in three wavelength bands
of, for example, 465.+-.12 nm, 520.+-.19 nm, and 635.+-.10 nm as
expressed in terms of the center wavelength combined with the
wavelength width at half the maximum intensity. In FIG. 3, the
intensity, taken along the vertical axis, is given in values
relative to the maximum intensity of B light taken as 100. The R,
G, and B light intensities of the light source 11 are adjusted with
consideration given to the diffraction efficiency of a HOE 16,
which will be described later, and the light transmittance of the
display element 13, and this enables display of white color.
[0041] The light source 11 is disposed in a positionally conjugate
relationship with the optical pupil E. This leads to high use
efficiency of the light from the light source 11 (allows the light
from the light source 11 to be incident on the optical pupil E
efficiently), and permits the viewer to observe a bright image. In
other words, it is possible to realize an image display device 1
with low power consumption. The image display device 1 may
incorporate a single set of light sources having R, G, and B
light-emitting portions respectively, or may incorporate two or
more such sets.
[0042] The illumination optical system 12 is an optical system that
directs the light from the light source 11 to the display element
13. In this embodiment, it is composed of a back-surface-reflection
mirror that has a refractive surface 12a on the front and a
reflective surface 12b on the back. The refractive surface 12a and
the reflective surface 12b are each a concave surface that has a
positive optical power on the YZ plane, that is disposed
eccentrically (decentered) with respect to the optical axis, and
that is concave toward the light source 11 and the display element
13. Specifically, the refractive surface 12a is a cylindrical
surface that has an optical power only on a plane parallel to the
YZ plane, and the reflective surface 12b is a cylindrical
aspherical surface that has an optical power only on a plane
parallel to the YZ plane. The refractive surface 12a and the
reflective surface 12b may each be a rotation-symmetric spherical
surface, rotation-symmetric aspherical surface, or a free-form
surface.
[0043] Between the illumination optical system 12 and the display
element 13, there may additionally be provided a unidirectional
diffuser plate that diffuses the incident light in one direction
(for example, the X direction). Providing the unidirectional
diffuser plate makes it possible, in a case where the set of light
sources having R, G, and B light-emitting portions which compose
the light source 11 are arranged in a row in the X direction, to
mix the R, G, and B light from the light source 11 in the X
direction. It is thus possible to reduce color unevenness due to
the light-emitting portions being arranged at different positions,
and it is also possible, owing to the diffusion by the
unidirectional diffuser plate, to enlarge the optical pupil E in
one direction.
[0044] In a case where a unidirectional diffuser plate is provided,
even when the light source 11 and the optical pupil E are disposed
in a positionally conjugate relationship with each other, they are
not optically conjugate with each other in the X direction, but are
still optically conjugate with each other in the Y direction. Thus,
in the Y direction, it is possible to direct the light from the
light source 11 to the optical pupil E efficiently.
[0045] Instead of the unidirectional diffuser plate mentioned
above, an ordinary diffuser plate may be provided that diffuses the
incident light in all directions. In this case, the position of the
diffuser plate may be taken as the light source position (the
secondary light source position), and this light source position
may be disposed in a positionally conjugate relationship with the
optical pupil E.
[0046] The display element 13 modulates the incident light
according to image data to display an image, and is composed of,
for example, a transmissive LCD. The display element 13 has a
rectangular display screen (image region), and is disposed with its
longer- and shorter-side directions aligned with the X and Y axes
respectively.
[0047] The eyepiece optical system 14 is an optical system which
directs the image light from the display element 13 to the optical
pupil E, and includes an eyepiece prism 15 that guides the image
light inside it. The eyepiece prism 15 has three optical surfaces,
namely surfaces S1, S2, and S3, and has a shape symmetric about the
YZ plane.
[0048] The surface S1 is an entrance surface through which the
image light enters. The surface S2 serves both as a total
reflection surface on which the image light is totally reflected
and as an exit surface through which the image light after being
diffraction-reflected by a HOE 16, which will be described later,
emerges toward the optical pupil E. The surface S2 is, for example,
a flat surface and is disposed on the optical pupil E side of the
surface S3. The surface S3 is a surface having a total reflection
surface and a HOE surface (the surface on which a HOE 16 is formed)
formed continuously, and is disposed opposite from the surface S2.
In this embodiment, the surface S3 is a surface having a curvature
only on the YZ plane. In this embodiment, the eyepiece prism 15 has
a tapering shape, meaning that it is so shaped that the distance
between the surfaces S2 and S3 continuously decreases away from the
surface S1. This shape will be described in detail later.
[0049] On part of the surface S3, a HOE 16 is formed which is a
volume-phase reflective holographic optical element. The HOE 16
directs the image light from the display element 13 to the optical
pupil E by diffraction-reflecting it. The HOE 16 has an
axis-asymmetric positive optical power, and functions in a similar
manner to an aspherical concave-surface mirror.
[0050] FIG. 1 is a sectional view showing, with enlargement, part A
in FIG. 2. The HOE 16 is produced by exposing a hologram
photosensitive material 16a to two beams (irradiating it with two
beams). In this embodiment, the hologram photosensitive material
16a is attached on the surface S3 in such a way that at least part
(for example, rays L1) of the beam of the image light totally
reflected on the surface S3 is incident on the attachment region R1
of the hologram photosensitive material 16a (meaning the region
across which the hologram photosensitive material 16a is attached).
So long as the just mentioned at least part of the image light is
incident within the attachment region R1 on the hologram
photosensitive material 16a, it does not matter whether that part
of the image light is incident on a region R2 or a region R3. The
region R2 is an effective diffraction region, which is the region
within the attachment region R1 where the HOE 16 is formed. On the
other hand, the region R3 is the region within the attachment
region R1 which is located outside the region R2. How the HOE 16 is
produced will be described in detail later.
[0051] FIG. 4 is a diagram illustrating the wavelength dependence
of the diffraction efficiency on the HOE 16. As shown there, the
HOE 16 is so produced as to diffract (reflect) light in three
wavelength bands of 465.+-.5 nm (B light), 521.+-.5 nm (G light),
and 634.+-.5 nm (R light) as expressed in terms of the diffraction
efficiency peak wavelength combined with the wavelength width at
half the diffraction efficiency peak level. A diffraction
efficiency peak wavelength here denotes the wavelength at which the
diffraction efficiency peaks, and a wavelength width at half a peak
diffraction efficiency level is the wavelength width within which
the diffraction efficiency is half its peak level or more. In FIG.
4, the diffraction efficiency is given in values relative to the
maximum diffraction efficiency for B light taken as 100.
[0052] As will be understood from FIGS. 3 and 4, the peak
wavelengths of the diffraction efficiency on the HOE 16 are
substantially equal to the peak wavelengths (center wavelengths) of
the intensity of the light emitted from the light source 11. This
allows, of the light emitted from the light source 11 (the light
constituting the image light), the parts at and around the
wavelengths at which its intensity peaks to be efficiently
diffracted on the HOE so as to be directed to the optical pupil
E.
[0053] Next, how the image display device 1 constructed as
described above operates will be described with reference to FIG.
2. The light emitted from the light source 11 is refracted at the
refractive surface 12a of the illumination optical system 12, is
then reflected on the reflective surface 12b, and is then refracted
again at the refractive surface 12a so as to be directed to the
display element 13. The light enters the display element 13, is
modulated while passing through it, and leaves it as image light.
The image light from the display element 13 then enters the
eyepiece prism 15 in the eyepiece optical system 14 through the
surface S1, is then totally reflected several times between the
surfaces S2 and S3, and is then incident on the HOE 16 on the
surface S3. Here, at least part of the image light totally
reflected from the surface S3 is incident on the attachment region
R1 (see FIG. 1) of the hologram photosensitive material 16a.
[0054] The light has to be totally reflected on the surface S3 at
least once. The image light from the surface S1 may, for example,
(1) be totally reflected on the surface S3, then totally reflected
on the surface S2, and then incident on the HOE 16 on the surface
S3, or (2) be totally reflected on the surface S2, then totally
reflected on the surface S3, then totally reflected on the surface
S2 again, and then incident on the HOE 16 on the surface S3.
[0055] The HOE 16 has such wavelength selectivity as to function as
a diffractive element only for light of wavelengths corresponding
to the emission wavelengths of the light source 11, and thus
functions as a concave reflective surface only for light of those
wavelengths. Accordingly, the light incident on the HOE 16 is
diffraction-reflected by it so as to reach the optical pupil E.
Thus, when the pupil P of a viewer is placed at the position of the
optical pupil E, the viewer can observe an enlarged virtual image
of the image displayed on the display element 13.
[0056] The HOE 16 only diffracts light of particular wavelengths
incident at particular angles of incidence, and therefore exerts
almost no effect on the transmission of outside light. Thus, while
observing the displayed image (virtual image), the viewer can also
observe an outside world image through the eyepiece prism 15 and
the HOE 16 on a see-through basis. Although the outside world image
suffers distortion as a result of its light being transmitted
through the eyepiece prism 15, this distortion can be corrected for
easily by attaching a correction prism 17 (see FIG. 9), which will
be described later, to the eyepiece prism 15.
[0057] This embodiment adopts a construction where the surface S3
has both a total reflection surface and a diffraction-reflection
surface (HOE surface), that is, a construction where a total
reflection surface and a HOE surface are formed on the same surface
S3. With this construction, as compared with one where those
surfaces are formed separately, it is possible to reduce the angle
of reflection on the HOE 16. Specifically, it is possible to set up
the HOE on the surface S3 parallel to the surface S2, and this
reduces the angle of incidence of the image light on the HOE 16;
thus it is possible to reduce the angle of reflection (diffraction)
on the HOE 16. By reducing the angle of diffraction on the HOE 16,
it is possible to keep the color dispersion caused by diffraction
small, and thus to maintain satisfactory image quality.
[0058] The eyepiece prism 15 is formed by the molding of resin such
as acrylic resin. For easier molding at that time, and for securer
attachment of the hologram photosensitive material 16a when it is
attached, it is necessary to secure a larger HOE surface. With a
construction where the HOE surface and the total reflection surface
are formed as separate surfaces, forming a larger HOE surface
results in making the eyepiece prism 15 thicker. By contrast, with
the construction according to this embodiment, it is possible to
form the HOE surface and the total reflection surface on the same
surface to set up the HOE surface. It is thus possible, while
avoiding making the eyepiece prism 15 thicker, to form a larger HOE
surface, and thereby to achieve easier prism molding and securer
attachment of the hologram photosensitive material 16a.
[0059] Moreover, in this embodiment, the eyepiece prism 15 has a
tapering shape, that is, it is so shaped that the distance between
the surfaces S2 and S3 continuously decreases away from the surface
S1. Thus, as light is reflected between the surfaces S2 and S3, its
angle of incidence with respect to the surface S2 or S3 decreases.
This produces a greater difference in angle of incidence with
respect to the surface S3 between a ray L1 that is reflected on the
surface S2 once and then incident on the hologram photosensitive
material 16a (surface S3) formed on the surface S3 and a ray that
is reflected on the surface S2 twice, then reflected on the surface
S3 once, and then incident on the HOE 16 (surface S3).
[0060] The volume-phase reflective HOE 16 has angle selectivity;
thus, even if part of the image light, which should ideally be
totally reflected, is incident on the region R2 within the
attachment region R1 of the hologram photosensitive material 16a,
that part of the image light is not readily diffraction-reflected
toward the optical pupil E. Actually, that part of the image light
is reflected on the HOE 16 (region R2) so as to be directed to the
surface S2, is then totally reflected on the surface S2, is then
incident back on the HOE 16 (region R2), and is
diffraction-reflected by it so as to be directed to the optical
pupil E. On the other hand, if part of the image light, which
should ideally be totally reflected, is incident on the region R3
within the attachment region R1 of the hologram photosensitive
material 16a, this part of the image light is totally reflected at
the interface with the layer of air. This part of the image light
is thereby directed to the surface S2, is then totally reflected on
the surface S2, is then incident on the HOE 16 (region R2), and is
diffraction-reflected by it so as to be directed to the optical
pupil E. In either case, it is possible to prevent any part of the
image light, which should ideally be totally reflected, incident on
the attachment region R1 of the hologram photosensitive material
16a from being readily diffraction-directed there so as to be
incident on the optical pupil E as ghost light.
[0061] It is thus no longer necessary, for the purpose of
preventing occurrence of ghost light, to secure an optical path
margin (a space for separating optical paths) between the
diffraction-reflection region of the HOE 16 and the total
reflection region for the image light, and this makes it possible
to make the eyepiece prism 15 accordingly slimmer. That is, with
the image display device 1 according to this embodiment, it is
possible to prevent occurrence of ghost light and in addition make
the eyepiece prism 15 slim and compact.
[0062] The hologram photosensitive material 16a is very thin, with
a thickness of, for example, 20 nm. Thus, even when the hologram
photosensitive material 16a partly overlaps the total reflection
region for the image light on the surface S3, this does not degrade
the optical performance of the image display device 1.
[0063] The HOE 16 in the eyepiece optical system 14 is used as a
combiner which directs the image light from the display element 13
and the light of the outside world image simultaneously to the
pupil P of a viewer. Thus, through the HOE 16, the viewer can
observe the displayed image on the display element 13 and the
outside image simultaneously. In particular, since the volume-phase
reflective HOE 16 has high wavelength selectivity and a narrow
reflection wavelength band, it is possible to present the viewer
with a bright, easy-to-see image even when superimposed on the
outside world image. Moreover, since the HOE 16 has an
axis-asymmetric positive optical power, it is possible to increase
flexibility in the arrangement of the individual optical members
constituting the device, and thereby to make the device compact
easily; in addition, it is possible to present the viewer with a
satisfactorily aberration-corrected image.
[0064] (Shape of the Eyepiece Prism)
[0065] Next, the shape of the eyepiece prism 15 will be described
in detail. FIG. 5 is a schematic sectional view of the eyepiece
prism 15. In this embodiment, as described above, the eyepiece
prism 15 has a tapering shape, that is, it is so shaped that the
distance between the surfaces S2 and S3 continuously decreases away
from the surface S1. Such a shape can be realized, for example, by
fulfilling conditional formulae (1) and (2) below.
d.theta./dy.gtoreq.0 (1)
d.sup.2.theta./ d.sup.2y.gtoreq.0 (2)
where
[0066] .theta. represents, for any point P on the YZ plane at which
a normal line T1 to the surface S2, which is a flat surface,
intersects the surface S3, the angle between the tangent line T2 to
the surface S3 at that point P and the normal line T1 to the
surface S2 (0.degree..ltoreq..theta..ltoreq.90.degree.); and
[0067] y represents the distance from the center of the optical
pupil E to the point P in the direction along the surface S2 (the Y
direction) on the YZ plane.
The variable .theta. is positive in the direction in which the
angle from the normal line T1 increases.
[0068] When conditional formulae (1) and (2) are fulfilled, the
point P on the surface S3 is located increasingly away from the
surface S2 as y increases, and thus the surface S3 is shaped such
that .theta. increases monotonously (a convex or flat surface). In
other words, the distance between the surfaces S2 and S3
continuously decreases away from the surface S1. In this way, it is
possible to set up an inclined HOE surface on the surface S3 and in
addition make the eyepiece prism 15 slim. An eyepiece prism 15 with
a flat surface S3 will be described later in connection with
Embodiment 3.
[0069] Consider two points Q1 and Q2 on the surface S2, the point
Q1 being closer to the surface S1; let the angle of incidence (in
terms of reverse tracing, the angle of total reflection) of the
axial ray (principal ray) with respect to the surface S2 at the
point Q1 be .phi.1(.degree.), and let the angle of incidence of the
principal ray with respect to the surface S2 at the point Q2 be
.phi.2(.degree.). Then, since the above-discussed shape of the
eyepiece prism 15 dictates that .phi.1>.phi.2, the display
element 13 can be disposed close to right above the surface S1 of
the eyepiece prism 15. This helps make the optical unit as a whole
slim.
[0070] In this embodiment, the surface S3 of the eyepiece prism 15
has a curvature only on the YZ plane; however, it may have a
curvature on the ZX plane as well. FIG. 6 is a perspective view of
an image display device 1 incorporating an eyepiece prism 15 having
a curvature on both of the just mentioned planes. Constructed in
this way, the image display device 1 offers further improved
optical performance (for example, aberration performance).
[0071] It is preferable that .phi.1 and .phi.2 be within the ranges
defined by conditional formulae (3) and (4) below.
50.degree.<.phi.1<70.degree. (3)
40.degree.<.phi.2<50.degree. (4)
[0072] By holding .phi.1 and .phi.2 less than or equal to their
respective upper limits, it is possible to prevent the eyepiece
prism 15 from being unduly long in the up/down direction. It is
then also possible to reduce the angle of incidence on the eyepiece
prism 15, and thereby to reduce the angle of diffraction on the HOE
16. This makes it possible to prevent image deterioration resulting
from occurrence of color dispersion caused by diffraction. On the
other hand, by holding .phi.1 and .phi.2 more than or equal to
their respective lower limits, it is possible to diminish the
overlap between the total reflection region on the surface S3 and
the diffraction region owing to the HOE 16. This makes it possible
to prevent image deterioration due to occurrence of ghost
light.
[0073] Table 1 lists the values of .phi.1 and .phi.2 as observed in
the image display device of Embodiment 1, and in those of
Embodiments 2 and 3, which will be described later. The listed
values indicate that the image display devices of all these
embodiments fulfill conditional formulae (3) and (4).
TABLE-US-00001 TABLE 1 Embod- Embod- iment 1 iment 2 Embodiment 3
Embodiment 3 (FIG. 2) (FIG. 10) (FIG. 11) (FIG. 12) .phi.1
(.degree.) 57.15 56.61 69.92 66.08 (50.degree. < .phi.1 <
70.degree.) .phi.2 (.degree.) 45.45 45.80 44.92 44.97 40.degree.
< .phi.2 < 50.degree.
[0074] (Method for Production of a HOE)
[0075] Next, how the HOE 16 mentioned above is produced will be
described. FIG. 7 is a sectional view showing an outline of the
construction of a production optical system for producing the HOE
16. The reflective HOE 16 is produced in the following manner: for
each of R, G, and B, a laser beam is split into two beams, called
the reference beam and the object beam respectively; a hologram
photosensitive material 16a on a substrate (here, the eyepiece
prism 15) is exposed, from both the substrate side and the opposite
side, with the two beams (reference and object beams) respectively;
by these two beams, interference fringes are recorded in the
hologram photosensitive material 16a. Now, a description will be
given of a specific method of producing the HOE 16. In the
description to follow, the beam from the side where a viewer's eye
is located is referred to as the reference beam, and the beam from
the opposite side is referred to as the object light; moreover, it
is assumed that the surface S3 of the eyepiece prism 15 is a
surface that has a curvature only on the YZ plane.
[0076] First, the hologram photosensitive material 16a is attached
to the surface S3 of the eyepiece prism 15. Usable as the hologram
photosensitive material 16a is a photopolymer, a silver halide
material, dichromated gelatin, or the like. Among these, a
photopolymer is preferable because it allows easy production by a
dry process.
[0077] Subsequently, in the production optical system, for each of
R, G, and B, a laser beam is split into two beams by a beam
splitter, and then the split beams (reference and object beams) are
each condensed to become a divergent beam diverging from a point
light source 21 or 22 respectively. The R, G, and B reference beams
are spherical waves emitted from point light sources 21 located at
an identical position, and are incident on the hologram
photosensitive material 16a from the eyepiece prism 15 side. Here,
the point light sources 21 for R, G, and B are located at the
center of the optical pupil E of the eyepiece optical system 14 as
it is during image observation. Instead, the point light sources 21
for R, G, and B may be arranged displaced from one another, but
still on the optical pupil E, with consideration given to
differences between the peak wavelengths of the light source 11
used during actual use and the emission wavelengths of the lasers
used during production, and with consideration given also to the
degree of contraction of the hologram photosensitive material 16a,
so that, during actual use, the light of the R, G, and B peak
wavelengths from the light source 11 (LEDs), after being diffracted
by the HOE 16, falls at the same position on the optical pupil
E.
[0078] On the other hand, the R, G, and B object beams are
divergent beams emitted from point light sources 22 located at an
identical position; these beams are shaped so as to have
predetermined wavefronts by a free-form-surface mirror 23, are then
reflected on a reflective mirror 24, and are then incident, through
a color correction prism 25, on the hologram photosensitive
material 16a from the side opposite from the eyepiece prism 15.
Here, the surface 25a of the color correction prism 25 is disposed
at such an angle as to cancel the chromatic aberration occurring
mainly due to the image light being refracted at the surface S1 of
the eyepiece prism 15 and at the surface S2 as the exit surface in
the eyepiece optical system 14 used during actual use. To prevent
ghosts resulting from surface reflection, it is preferable that the
color correction prism 25 be disposed either in close contact with
the hologram photosensitive material 16a or with a medium, such as
emulsion oil, having the same index of refraction as the color
correction prism 25 interposed in between.
[0079] Through the irradiation of the hologram photosensitive
material 16a with (its exposure to) the reference and object beams
as described above, interference fringes are recorded in the
hologram photosensitive material 16a by those two beams, and in
this way, the HOE 16 is produced.
[0080] At this time, the reference and object beams have their
respective beam shapes restricted by beam restricting plates 31 and
32 so as to strike only the regions on the hologram photosensitive
material 16a where to record the hologram (interference fringes).
Accordingly, the formation region of the HOE 16 (meaning the region
across which the HOE 16 is formed, corresponding to the region R2
in FIG. 1) on the surface S3 is smaller than the attachment region
of the hologram photosensitive material 16a (corresponding to the
attachment region R1 in FIG. 1).
[0081] As described above, the effective diffraction region within
the attachment region of the hologram photosensitive material 16a
where the HOE 16 is produced (the formation region of the HOE 16)
is set by restricting the exposed region within the attachment
region. This makes it possible to attach a hologram photosensitive
material 16a larger than the effective diffraction region to the
surface S3 and then restrict the exposed region, thereby to form
the HOE 16 in a desired position. Consequently, it is possible to
alleviate the positioning accuracy with which the hologram
photosensitive material 16a needs to be attached on the surface S3.
Moreover, by inserting the beam restricting plates 31 and 32 in the
optical path of the production optical system and thereby
restricting the beam diameters of the two beams to which the
hologram photosensitive material 16a is exposed, it is possible to
restrict the exposed region easily and accurately.
[0082] Moreover, since the surface S3 of the eyepiece prism 15 has
a curvature only on the YZ plane, it is possible to attach the
hologram photosensitive material 16a in sheet form to the surface
S3 easily, thereby to produce the HOE 16. This makes the production
of the HOE 16 easy.
[0083] (Another Construction of the Image Display Device)
[0084] FIG. 8 is a sectional view showing another construction of
the image display device 1. As shown there, in the image display
device 1, the attachment region R1 of the hologram photosensitive
material 16a may include all of the region R2 as the
diffraction-reflection region and a total-reflection region R4 for
the image light on the surface S3.
[0085] In this case, the hologram photosensitive material 16a is so
large as to include both of the regions R2 and R4, and therefore
these two regions R2 and R4 are optically continuous at their
border. Thus, the viewer can observe a satisfactory image all
across the screen (image region). Also when observing an outside
world image on a see-through basis, the viewer observes it through
the hologram photosensitive material 16a (including the HOE 16) all
across the field of view, and thus the viewer can observe the
outside world image as a uniform image (not as a discontinuous
image).
[0086] (Yet Another Construction of the Image Display Device)
[0087] FIG. 9 is a sectional view showing yet another construction
of the image display device 1. As shown there, in the image display
device 1, the eyepiece optical system 14 may further include a
correction prism 17 and a positioning portion 18.
[0088] The correction prism 17 is a prism for canceling the
refraction of the light of the outside world image at the eyepiece
prism 15. The positioning portion 18 is a projection (spacer) for
joining together the eyepiece prism 15 and the correction prism 17
at a predetermined interval from each other with a layer of air in
between, and is formed on at least one of the eyepiece prism 15 and
the correction prism 17.
[0089] In particular, the eyepiece prism 15 and the correction
prism 17 are joined together with two positioning portions 18
interposed in between in such a way that a layer of air is formed
between the total reflection region for the image light on the
surface S3 of the eyepiece prism 15 and the surface 17a of the
correction prism 17 facing the surface S3, and that a layer of air
is also formed between the attachment region of the hologram
photosensitive material 16a and the surface 17a. Here, the joint
lines B1 and B2 along which the eyepiece prism 15 and the
correction prism 17 are joined together are located on side
surfaces that intersect the surfaces (for example, the surfaces S2
and S3) through which the light of the outside world image is
transmitted.
[0090] In a case where, as shown in FIG. 9, the eyepiece prism 15
is so shaped as to be increasingly thin away from the surface S1,
due to the light of the outside world image being refracted at the
surfaces S2 and S3, the outside world image observed through the
eyepiece prism 15 suffers distortion. By joining, however, the
correction prism 17 to the eyepiece prism 15 with a layer of air
and a positioning portion 18 interposed in between to substantially
form a parallel plate as a whole, and allowing observation of the
outside world image through the eyepiece prism 15 and the
correction prism 17, it is possible to prevent the observed outside
world image from suffering distortion.
[0091] Moreover, in the eyepiece optical system 14, all the joint
lines B1 and B2 are located on surfaces that intersect the surfaces
through which the light of the outside world image is transmitted,
and thus, when the outside world image is observed on a see-through
basis, the joint lines B1 and B2 are located outside the field of
view. This permits the viewer to observe the outside world image
satisfactorily. Moreover, the eyepiece prism 15 and the correction
prism 17 then have a flat part at the tip end. This makes the
molding of these prisms easy, makes their attachment easy, and thus
helps reduce cost.
[0092] Moreover, owing to the positioning portion 18, the eyepiece
prism 15 and the correction prism 17 can be kept at a predetermined
interval from each other with a layer of air in between. This makes
it possible to ensure that the image light is totally reflected
inside the eyepiece prism. In particular, by providing a layer of
air also between the attachment region of the hologram
photosensitive material 16a and the surface 16a of the correction
prism 17, even if part of the image light, which should ideally be
totally reflected, is incident on a region within the attachment
region of the hologram photosensitive material 16a but outside the
effective diffraction region, it is possible to ensure that that
part of the image light is totally reflected at the interface with
the layer.
Embodiment 2
[0093] Another embodiment of the invention will be described below
with reference to the accompanying drawings. For convenience' sake,
in the following description, such parts as are found in Embodiment
1 will be identified with the same reference signs, and no
overlapping description will be repeated.
[0094] FIG. 10 is a sectional view showing an outline of the
construction of an image display device 1 according to this
embodiment. In the image display device 1 of this embodiment, two
kinds of HOE are produced on the surface S3 of the eyepiece prism
15 in the eyepiece optical system 14, and with these two kinds of
HOE in between, the eyepiece prism 15 and the correction prism 17
are joined together. The two kinds of HOE are both volume-phase
reflective HOEs.
[0095] Of those HOEs, one will be referred to as the first HOE 41,
and the other will be referred to as the second HOE 42. The second
HOE 42 is produced by exposing a hologram photosensitive material
42a attached over the entire surface S3 to the two beams. The first
HOE 41 too is produced by exposing the hologram photosensitive
material 42a to the two beams. Thus, the attachment region R1 of
the hologram photosensitive material 42a where the second HOE 42 is
produced includes a diffraction-reflection region R6 of the second
HOE 42 and a diffraction-reflection region R5 of the first HOE
41.
[0096] Moreover, in this embodiment, as shown in FIG. 10, the
diffraction-reflection region R6 of the second HOE 42 and the
diffraction-reflection region R5 of the first HOE 41 partly
overlap. That is, in part of the hologram photosensitive material
42a, interference fringes for the first HOE 41 and interference
fringes for the second HOE 42 are both formed by multiple exposure.
Thus, part of the beam of the image light diffraction-reflected by
the first HOE 41 is incident also on the diffraction-reflection
region R6 of the second HOE 42.
[0097] In the construction described above, the image light from
the display element 13 enters the eyepiece prism 15 through the
surface S1, is then diffraction-reflected by the first HOE 41 on
the surface S3 at least once, is then totally reflected on the
surface S2, and is then diffraction-reflected by the second HOE 42
on the surface S3 so as to be directed to the optical pupil E. With
this construction, where the surface S3 has a
diffraction-reflection surface (first HOE surface) owing to the
first HOE 41 and a diffraction-reflection surface (second HOE
surface) owing to the second HOE 42, that is, where two HOE
surfaces are formed on the same surface S3, it is possible to set
up a second HOE surface in a direction parallel to the surface S2.
This reduces the angle of incidence of the image light on the
second HOE 42, and thus makes it possible to reduce the angle of
reflection (diffraction) on the second HOE 42. By reducing the
angle of diffraction on the second HOE 42, it is possible to reduce
the color dispersion caused by diffraction, and thus to maintain
satisfactory image quality.
[0098] Moreover, since, as in Embodiment 1, the distance between
the surfaces S2 and S3 continuously decreases away from the surface
S1, even with a construction where part of the beam of the image
light diffraction-reflected by the first HOE 41 on the surface S3
is incident on the diffraction-reflection region R6 of the second
HOE 42, the part of the image light that is incident on the
diffraction-reflection region R6 of the second HOE 42 despite the
fact that the image light should ideally be diffraction-reflected
(for example, regularly reflected) by the first HOE 41 can be
diffraction-reflected (for example, diffracted at an angle of
reflection close to that for regular reflection) on the
diffraction-reflection region R6 of the second HOE 42. That is,
since a volume-phase reflective HOE has angle selectivity, even if
part of the image light, which should ideally be totally reflected,
is incident on the second HOE 42, that part of the image light is
not diffraction-reflected by it toward the optical pupil E. It is
thus no longer necessary, for the purpose of preventing occurrence
of ghost light, to secure an optical path margin (a space for
separating optical paths) between the diffraction-reflection region
R5 of the first HOE 41 and the diffraction-reflection region R6 of
the second HOE 42, and this makes it possible to make the eyepiece
prism 15 accordingly slimmer. Thus, with the construction described
above, it is possible to prevent occurrence of ghost light and in
addition make the eyepiece prism 15 slim and compact.
[0099] Moreover, the attachment region R1 of the hologram
photosensitive material 42a, where the second HOE 42 is produced,
includes both the diffraction-reflection region R6 of the second
HOE 42 and the diffraction-reflection region R5 of the first HOE
41, and the hologram photosensitive material 42a is so large as to
include both of the diffraction-reflection regions R5 and R6; thus,
these regions are optically continuous at their boundary. Thus, the
viewer can observe a satisfactory image all across the screen
(image region). Also when observing an outside world image on a
see-through basis, the viewer observes it through the hologram
photosensitive material 42a (including the diffraction-reflection
regions R5 and R6) all across the field of view, and thus the
viewer can observe the outside world image as a uniform image (not
as a discontinuous image). Furthermore, when the correction prism
17 is attached to the eyepiece prism 15, it is possible to join the
eyepiece prism 15 and the correction prism 17 together with no
layer of air in between, and thus to join them together stably.
[0100] Moreover, in part of the hologram photosensitive material
42a, interference fringes for the first HOE 41 and interference
fringes for the second HOE 42 are both formed by multiple exposure.
Thus, even if part of the beam of the image light
diffraction-reflected by the first HOE 41 is incident on the
diffraction-reflection region R6 of the second HOE 42, it is
possible to ensure that that part of the image light is
diffraction-reflected (for example, diffracted at an angle of
reflection close to that for regular reflection) by the
interference fringes of the first HOE 41. Moreover, by making the
angle of diffraction on the first HOE 41 close to that for regular
reflection, it is possible to suppress occurrence of color
dispersion.
[0101] In this embodiment, one kind of hologram photosensitive
material, that is, the hologram photosensitive material 42a for
producing the second HOE 42, is subjected to two types of exposure
to produce two kinds of HOEs (the first and second HOEs 41 and 42).
Instead, it is also possible to prepare two kinds of hologram
photosensitive material, then attach one hologram photosensitive
material to the surface S3 and expose it to produce the second HOE
42, then treat it by a fixing process, and thereafter attach the
other hologram photosensitive material to the surface S3 and expose
it to produce the first HOE 41. Here, it is also possible to attach
the two hologram photosensitive materials to the surface S3 in such
a way that they party overlap and expose them to produce the two
kinds of HOE.
Embodiment 3
[0102] Yet another embodiment of the invention will be described
with reference to the accompanying drawings. For convenience' sake,
in the following description, such parts as are found in Embodiment
1 or 2 will be identified with the same reference signs, and no
overlapping description will be repeated.
[0103] FIG. 11 is a sectional view showing an outline of the
construction of an image display device 1 according to this
embodiment. The image display device 1 of this embodiment has a
similar construction to that of Embodiment 2, the differences being
that the eyepiece prism 15 has a flat surface as the surface S3,
has a surface S4 substantially parallel to the surface S2, and has
the surfaces S1 and S3 connected together by the surface S4 outside
the effective optical path region of the image light. The
correction prism 17 is here so disposed that the surface 17a only
faces the surface S3 across the two kinds of HOE.
[0104] By making the surface S3 flat, it is possible to make the
surface 17a of the correction prism 17 facing the surface S3 flat,
and thereby to simplify the structures of the eyepiece prism 15 and
the correction prism 17. Moreover, if, for example, the surface S3
and the surface 17a are both curved surfaces, the eyepiece prism 15
and the correction prism 17 may make partial contact with each
other when joined together; by contrast, with the surface S3 and
the surface 17a both being flat surfaces, when the eyepiece prism
15 and the correction prism 17 are joined together, even if the
interval between them is small, it is possible to join them
together while preventing them from making partial contact with
each other. This makes joining the eyepiece prism 15 and the
correction prism 17 together easy.
[0105] Moreover, connecting the surface S4 of the eyepiece prism 15
to the surface S3 outside the effective optical path region of the
image light permits the surfaces S2 and S4 to be parallel outside
the effective optical path region, and this makes it possible to
make the eyepiece prism 15 slim.
[0106] FIG. 12 is a sectional view showing another construction of
the image display device 1. This image display device 1 has a
combination of the above-described construction shown in FIG. 9 and
the construction shown in FIG. 11 having a flat surface as the
surface S3, with the positioning portion 18 omitted and the
correction prism 17 given a slightly modified shape. That is, the
surface S3 of the eyepiece prism 15 and the surface 17a of the
correction prism 17 are flat surfaces, and the correction prism 17
has a positioning portion 19. The positioning portion 19, when the
eyepiece prism 15 and the correction prism 17 are joined together,
makes contact with the surface S4 of the eyepiece prism 15 outside
the total reflection region and thereby serves to position the
correction prism 17 relative to the eyepiece prism 15. The
positioning portion 19 extends from the correction prism 17
parallel to the surface S4.
[0107] With this construction, by putting the positioning portion
19 of the correction prism 17 in contact with the surface S4 of the
eyepiece prism 15, it is possible to achieve positioning easily.
Moreover, the joint line between the eyepiece prism 15 and the
correction prism 17 is located on the same surface as the surface
Si and hence outside the observation region of the outside world
image; thus, the viewer can observe the outside world image
satisfactorily.
Embodiment 4
[0108] Still another embodiment of the invention will be described
with reference to the accompanying drawings. For convenience' sake,
in the following description, such parts as are found in any of
Embodiments 1 to 3 will be identified with the same reference
signs, and no overlapping description will be repeated.
[0109] FIG. 13 is a perspective view showing an outline of the
construction of a HMD according to this embodiment. This HMD is
composed of an image display device 1 according to any of the
embodiments described previously and a support member 2.
[0110] The image display device 1 has a light source 11 and a
display element 13 (see FIG. 1) housed in a housing 3, and has an
eyepiece optical system 14 integrated with the housing 3. The
signals and supply electric power for controlling the light source
11 and the display element 13 are fed to their respective
destinations via a cable 4 that penetrates the housing 3. The
eyepiece optical system 14 is as a whole shaped like one (in FIG.
13, the one for the right eye) of the lenses of spectacles
(eyeglasses). As a lens 5 corresponding to the other, for the left
eye, of the lenses of spectacles, a dummy lens is provided.
[0111] The support member 2 serves as a supporting means whereby
the image display device 1 is supported in front of an eye of a
viewer, and is composed of, for example, a set of members
corresponding to the frame and temples of spectacles. When the
support member 2 is fixed on the viewer's head, the image display
device 1 is held in an accurate position in front of his eye; thus,
the viewer can observe the image presented by the image display
device 1 in a hands-free fashion stably for a long time. In
particular, according to the present invention, it is possible to
make the eyepiece prism 15 in the eyepiece optical system 14 slim
and compact, and thus to realize a compact, lightweight HMD.
Whereas in this embodiment the support member 2 supports one image
display device 1 corresponding to the viewer's right eye, it may
instead support two image display devices corresponding to both
eyes of the viewer.
[0112] The support member 2 has a fixing mechanism 6. The fixing
mechanism 6 serves as a fixing means whereby, after the position of
the optical pupil E is adjusted to the viewer's pupil P (anatomical
pupil, iris), the eyepiece optical system 14 is kept in a fixed
position relative to the viewer's head. The fixing mechanism 6 is
composed of a right nose pad 6R and a left nose pad 6L, which
movably make contact with the viewer's nose, and a locking portion
which locks them. Owing to the support member 2 having the fixing
mechanism 6, after the position adjustment of the optical pupil,
the viewer can observe a satisfactory image at the position of the
optical pupil without fail and stably for a long time.
[0113] Although the embodiments deal with examples where the light
source 11 is composed of LEDs, the light source 11 may be a laser
light source. Using a laser light source makes it possible to
eliminate the effect of the dispersion caused by diffraction on a
HOE, and thus permits the viewer to observe a high-quality, bright
image.
[0114] Needless to say, it is possible to build an image display
device 1, and hence a HMD, by combining features from different
embodiments together appropriately.
[0115] Any of the image display devices 1 described above as
embodiments may also be applied to, for example, a head-up display
(HUD).
INDUSTRIAL APPLICABILITY
[0116] The present invention find applications in HMDs and
HUDs.
LIST OF REFERENCE SIGNS
[0117] 1 image display device [0118] 2 support member (support
means) [0119] 13 display element [0120] 14 eyepiece optical system
[0121] 15 eyepiece prism [0122] 16 HOE [0123] 16a hologram
photosensitive material [0124] 17 correction prism [0125] 18
positioning portion [0126] 19 positioning portion [0127] 41 first
HOE [0128] 42 second HOE [0129] 42a hologram photosensitive
material [0130] E optical pupil [0131] R1 attachment region [0132]
R2 region (effective diffraction region) [0133] R3 region [0134] R4
total-reflection region [0135] R5 diffraction-reflection region
[0136] R6 diffraction-reflection region [0137] S1 surface [0138] S2
surface [0139] S3 surface
* * * * *